An effective approach to linear and angular position determining systems has been shown in my previously issued U.S. Pat. No. 4,035,762, issued on July 12, 1977. Such patent describes various embodiments for measuring a position along a line, either straight or curved, or on a plane or other curvilinear surface, with a high degree of accuracy at a reasonable cost. The embodiments described therein generally require two relatively movable members. In a particular embodiment, for example, one member may comprise a driver transducer and a delay line, both fixedly positioned, and a second member may comprise a receiver transducer which is movable with respect to the fixed driver transducer and the delay line. The resolution of the displacement measurement depends upon the phase shift experienced by a continuous elastic wave traveling along the delay line element in a direction parallel to, or coincident with, the distance to be measured.
Various embodiments of the above concept are described in such issued patent. Further inventive position determining systems have also been described in my previously filed patent applications: Ser. No. 747,072, filed Dec. 2, 1976, now U.S. Pat. No. 4,121,155; Ser. No. 803,807, filed June 6, 1977, now U.S. Pat. No. 4,144,519; and Ser. No. 842,052, filed Oct. 14, 1977, now U.S. Design Patent D.252,775. All of said issued patent and patent applications are incorporated herein by reference.
Other position determining systems of various types have also been suggested in the past in addition to, and different from, those set forth in my above-referred to patents and patent applications. For example, previous systems have been described in U.S. Pat. No. 3,121,955, "Ultrasonic Distance Scaling Apparatus," issued to K. L. King on Feb. 25, 1961 and in the article "Precision Micropositioning Using Acoustic Surface Wave Variable Delay Lines," J. P. Baker et al., Review of Scientific Instruments, Vol. 45, No. 1, 1974.
In all of such systems a suitable signal generator for providing an excitation signal at a selected wavelength is utilized for supplying an electrical signal to a driver transducer which in turn produces the traveling elastic wave. Such signal generator is generally in the form of precision oscillator circuitry, the need for which represents additional costs and increased physical space requirements for the overall system. Moreover, temperature variations cause the velocity of propagation of the traveling wave to change in accordance therewith and a temperature compensation means, such as a phase-locked loop as described therein, is required to vary the frequency of the drive signal in order to compensate for such propagation velocity variations so as to maintain the same wavelength in the face of temperature variations. Although the use of the phase-locked loop approach permits compensation for all factors effecting the phase of the received signal, whether arising from temperature effects on the medium or from other sources, the need for such a phase-locked loop also represents an increase in the cost and spatial requirements of the overall position determining system.
It is desirable, therefore, in certain applications to eliminate the need for a separate external precision oscillator signal source and to provide for compensation of temperature variations, at least in the delay medium, in an effective manner which permits a simplification of the structure thereof as much as possible and a reduction in the overall cost while, at the same time, still providing high resolution position determination.
The generation of oscillating signals in delay media in general is, of course, known. For example, surface elastic wave oscillator devices for such purpose have been suggested in the past. Exemplary oscillators of such type have been described in "Surface-Acoustic-Wave Oscillators," J. Crabb et al., Electronics Letters, 17 May 1973, Vol. 9, No. 10, pp. 195-197; "Improvements to the SAW Oscillator," R. Bale et al., IGEE Cat. No. 74, CHO 896-ISU, pp. 272-275, 1974 Ultrasonics Symposium Proceedings. Further, an oscillator used in determining temperatures of a traveling strip of metal has been discussed in U.S. Pat. No. 3,186,226, "Apparatus for Determining the Temperature of Traveling Strip," issued on June 1, 1965, to J. S. Milner et al.
None of such devices, however, have been used in position determining systems nor has any technique for their use in such a context ever been previously suggested.